Elevation and climb data

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Elevation and climb data

Postby gerhard » Fri May 01, 2009 3:46 am

This section explains some aspects to Elevation, ascent/descent and smoothing in SportTracks

Question:

What elevation source is best?

Answer:

However, most of the differences comes from the measurement methods:
* Map contours is normally best, if the contours are close enough compared to the "hillyness".
* Barometric elevation is the second best, like in Garmin Edge 305/705. (See the post below for Garmin notes)
* Elevation Correction with SRTM data is like the Map Contours, but samples are done in a "matrix" pattern. Depending on the terrain, the path taken may not be representative. See the post below for details.
* GPS elevation as in most GPS devices. Very noisy, requires a lot of smoothing to come close to a value that can be compared to other methods.

A comparison digital maps vs. barometric+GPS (Edge 305) vs. GPS only (FR 305) is available here.
An example picture from the post (no smoothing applied):
Image
Last edited by gerhard on Thu May 14, 2009 2:24 pm, edited 3 times in total.
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Postby gerhard » Fri May 01, 2009 4:08 am

Question:

What elevation settings are recommended?
Why does the ascend/descend data differ from Garmin Training Center?

Answer:

There is no true data to ascend/descend. Should small hills where you keep the momentum be included? False long flats?
It is hard to make algorithms that includes all situations and yet filters the noise in the data. You have to draw the limit at one pint what to include. If you go down in detail you could include bumps and gravel.

To analyze data you need to apply smoothing, as the data generally has a lot of noise. For instance Garmin Training Center does that too, but the algorithm is unknown.

The settings that allows for some type of comparison between devices and activities, depends to most extent on the elevation source:
* Barometric elevation and good digital maps: Flat zone +/- 1%, 15 sec smoothing
* Elevation Correction with SRTM data: Flat +/- 2%, 30 sec smoothing
* GPS elevation: Flat +/- 2%, 60 sec smoothing The GPS elevation data should have even higher smoothing to be compared to other, but then you loose the contours.

The recommended settings depends also of the type of the activity, how frequent elevation changes occurs. See the post below for further explanation:
http://www.zonefivesoftware.com/SportTr ... 0061#30061

The setting can be "calibrated" with a safe source of data. An example, applicable with barometric elevation. If you own or can borrow a "good" digital map (1:25000, 1:50000) with elevation information, a good approach to determine your settings yourself is:
- Choose a representative set of your activities (mainly flat, with medium climb, with a lot of climb)
- Now determine the ascent/descent for these activities with the digital map. The map will sum up "everything", regardless of the steepness.
- Then in SportTracks in Select View=>categories=>Climb Zones set up your "flat" zone to its minimum (-0.1% to 0.1%), it will also sum up "everything"
- Now change the smoothing setting until you get the best match with the results from the digital maps
- Once done, you still can change your "flat" zone. What steep grades to include in your total ascent/descent is more a philosophical than a "scientifical" question.

Some Edge units record climb section preciser than flat sections.
If your "calibration" as explained above fits well the chosen activities with medium and much climb, but not as good on your "flat" activities, you could modify the "calibration" as follows. Keep your established smoothing setting for medium/high climb, then try to compensate the errors in the flat by adjusting the boundary settings of your "flat" zone.
Last edited by gerhard on Thu May 14, 2009 2:34 pm, edited 2 times in total.
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Postby gerhard » Fri May 01, 2009 4:30 pm

Question:

Garmin (barometric) devices specific notes?

Answer:

* There is a "noise" from the measurements, the value can toggle +/- 0.48 m on flats. Therefore one second recording gives inflated values. See this thread going into detail. It is established that Edge 305&705, Forerunner 305&405 do record the elevation in steps of approx. 0.481 meters. ALL elevation differences in the ORIGINAL record from one point to the next will be 0 OR a multiple of 0.481 meters. Due to this, notably grade values and graphs might be very inaccurate locally, however, they get reasonable if averaged over some time.
* Barometric changes may give absolute differences (so passing a certain point gives different values each time). The device auto calculates during the ride, but if the pressure changes fast enough, the device will not calibrate enough.
* Beware of the "kludge" when starting - the device syncs the barometric elevation to the GPS height then.
* An elevation "jump" may be observed with an Edge 305 at start of a record. This thread goes into details.
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Postby gerhard » Fri May 01, 2009 4:35 pm

Question:

Which settings in SportTracks do affect the calculation of total ascent/descent?


Answer:

  • Your choice in Settings=>Analysis=>Data Smoothing=>Elevation . You can turn smoothing off completely (box unchecked) or turn it on and choose a smoothing "value". The "value" is shown when pointing to the slider, so it is possible to reproduce a chosen smoothing.
  • Your choice in Select View=>Categories=>Climb Zones. Whatever zones you choose, there is always a "Flat" zone. Its minimum setting is -0.1% to 0.1%. All ascent and descent that falls into your chosen "Flat" zone is NOT taken into account for the calculation of total ascent/descent.


Generally fast small changes (like small hills) are better tracked with less smoothing and larger "flat" zone.
Similarly, small gradual changes requires more smoothing and smaller "flat" zone.

You can partly adapt to different types of activities with separate Climb Zones, but the Smoothing is the same for all activities, so you need to adjust that manually.

Thanks to CHnuschti for contributing to this entry.
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Postby gerhard » Sat May 02, 2009 3:48 pm

Question:

How accurate is elevation data I can get from internet sources and/or SRTM?

Answer:

See also the Elevation Correction Plugin Home Page available on the Elevation Correction Plugin Download Page

There are many sources available on the internet, where you can get information about altitude and/or download the altitude for a given track.

SRTM is a global altitude database. It covers a lot of our planet, and is publicly available for free. Its resolution is better in the USA than in the rest of the world. For instance, it is the unique altitude database with global coverage available. Other altitude sources usually are of national property, cover only a nation and are subject to license fees etc.. (PS: An exception seems to be NED for the USA, publicly available for free an of good accuracy). Therefore, most online sources offering altitude information very likely use the SRTM database, although they might not clearly declare it.

SRTM altitude data is fairly accurate in the open field, but since its data survey was done with radar, it reproduces the elevation of the surface. Therefore, its data can be incorrect, mainly in the woods, and, to a lesser grade, in heavily built-up areas.
The picture shows a comparison between SRTM-altitude and the altitude given by a digital map along a track.
Image

Thanks to CHnuschti for contributing to this entry.
gerhard
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